1. Field of the Invention
The invention relates to a heat discharging structure for use with a semiconductor device.
2. Description of Related Art
In general, when electronic parts such as semiconductor elements are operated, heat is generated through the consumption of electric power. When the temperature becomes too high, the functions and characteristics of the electronic parts are impaired, and the product life of the electronic parts is shortened. For example, in the case of a complementary metal oxide semiconductor (CMOS) integrated circuit, when the temperature of the environment or the part itself exceeds a predetermined level, the response speed of the transistor elements drops sharply, and properties such as operational clock frequency and access time are affected.
In order to discharge to the outside the heat arising from the electronic parts, it is common to use methods such as making a slit in the casing of the electronics to allow outside air to enter naturally, and forcing cooling of the inside by using a fan to blow air in or exhaust air out. In addition, in the case of a semiconductor device which produces large amounts of heat, such as a central processing unit (CPU), there have been innovations such as increasing the heat discharging area by installing heat sinks, using heat discharging substrates such as fins, or cooling by installing heat pipes, such as water cooling jackets.
For example, as shown in the prior art of FIG. 10, heat discharging fin 1 is attached by means of heat conducting adhesive agent 5 to the upper face of resin package 4 of semiconductor chip 3, which is mounted on circuit board 2. Also, as shown in the small notebook-type computer 6 of the prior art in FIG. 11, the heat discharging substrate 8 inside of case 7 is made so as to connect to the surface of package 11 of CPU 10, mounted on circuit board 9, so that heat from CPU 10, which consumes the most electric power, is conducted to the outside by heat discharging substrate 8, without adversely affecting the properties of the CPU, thus preserving the functions of the computer.
Heat arising from electronic parts can also be released to the outside from the outer lead through the wiring pattern of the circuit board. For example, in the examples of prior art in FIGS. 10 and 11, heat is conducted from respective bonding wires 12 and 13 through respective outer leads 14 and 15 to respective circuit board wiring patterns 16 and 17.
In addition, in the case of surface mounting by wire bonding and chip on board (COB) systems, because the heat conductivity of the bonding material generally used to resin-seal semiconductor parts is low, there is a method known which involves making thermal via holes in the circuit board and discharging heat to the outside from the conductor pattern of the inner layer, for example the ground layer or electronic power supply layer. Also, in a tape carrier package (TCP) system or the COB system mentioned above, the method is also used of decreasing heat production as much as possible by operating the semiconductor element at low voltage and reducing electric current consumption.
On the other hand, in the case of so-called Integrated Circuit (IC) cards, which enclose circuit boards mounted with electronic parts within card-shaped metal-construction cases as explained in Japanese laid-open patent publication number Hei 3-182397, a structure is suggested wherein the semiconductor element, which packages the semiconductor chip, is mounted on the circuit board, and heat arising from the semiconductor element is discharged by passage through a high heat conductive material, which fills the space between the semiconductor element and the external metallic panel, to the outside from the external metallic panel. In addition, in the IC card listed in the specifications for Japanese laid-open utility model publication Hei 4-7175, the effectiveness of heat discharging is heightened by making heat discharging fins on the outside face of the metallic panel.
Because the circulation fans, heat sinks, heat pipes, and other such heat discharging methods of the prior art as explained above increase the size of the device as a whole, they are not well-suited to a small semiconductor device. The heat discharging fins in FIG. 10 are expensive, and, in addition, manufacturing costs are high because of the added assembly steps needed to install them on semiconductor parts.
Further, small-size semiconductors used in portable information equipment such as notebook computers and electronic notebooks have a high probability of receiving shocks to their mechanisms due to being dropped or other incidents of daily use, which may cause the adhesive agent of the heat discharging fins to come off, impairing heat discharging capability and increasing the risk of possible damage to machine functioning due to accumulated internal heat.
Also, with semiconductor parts such as the CPU 10 shown in FIG. 11, generally the heat resistance is high for the mold resin forming package 11, and it is very thick, being several hundred .mu.m (micron meters) in order to protect the CPU, thereby raising heat resistance still more. For this reason, the structure of the prior art shown in this drawing, consisting only of using heat discharging substrate 8 to release heat from the top face of package 11, does not afford a sufficient degree of heat discharge.
The same is true of the IC card detailed in the above mentioned Japanese laid-open patent publication number Hei 3-182397, wherein heat is dispersed from only one surface of the semiconductor element package. With this type of small semiconductor device, achieving a strategy for heat discharge is an important element in promoting greater efficiency and function.
Natural convection through a slit in the structure has the advantage of being a simple change that does not enlarge the device as a whole, but it provides only a low heat discharging effect. In addition, particularly with small items for everyday use such as IC cards, this method is not preferred because it lowers the dustproofing and waterproofing properties so important in the use of these products.
With surface mounting such as in the COB system, the structure wherein thermal via holes alone constitute a heat discharging route cannot be expected to produce a sufficient heat discharging effect. Indeed, if the number of thermal via holes is increased, the area of the semiconductor pattern of the inner layer is correspondingly decreased, so heat conducting efficiency naturally decreases.
When a semiconductor element is operated at low voltage in order to control the amount of heat arising from consumption of electric current, there is the danger of reducing the performance characteristics of the semiconductor element such as clock frequency and access time. This, in turn, decreases the efficiency and performance of the semiconductor device.